Kristopher Johnson kristopherjohnson

libdispatch efficiency tips

The libdispatch is one of the most misused API due to the way it was presented to us when it was introduced and for many years after that, and due to the confusing documentation and API. This page is a compilation of important things to know if you're going to use this library. Many references are available at the end of this document pointing to comments from Apple's very own libdispatch maintainer (Pierre Habouzit).

My take-aways are:

You should create very few, long-lived, well-defined queues. These queues should be seen as execution contexts in your program (gui, background work, ...) that benefit from executing in parallel. An important thing to note is that if these queues are all active at once, you will get as many threads running. In most apps, you probably do not need to create more than 3 or 4 queues.
Go serial first, and as you find performance bottle necks, measure why, and if concurrency helps, apply with care, always validating under system pressure. Reuse

A composable pattern for pure state machines with effects

State machines are everywhere in interactive systems, but they're rarely defined clearly and explicitly. Given some big blob of code including implicit state machines, which transitions are possible and under what conditions? What effects take place on what transitions?

There are existing design patterns for state machines, but all the patterns I've seen complect side effects with the structure of the state machine itself. Instances of these patterns are difficult to test without mocking, and they end up with more dependencies. Worse, the classic patterns compose poorly: hierarchical state machines are typically not straightforward extensions. The functional programming world has solutions, but they don't transpose neatly enough to be broadly usable in mainstream languages.

Here I present a composable pattern for pure state machiness with effects,

I’ve been writing a load of Swift code recently for work and this has lead me into the world of typed functional programming. The app needs to build certain objects from a comma separated string, and this lead me to applicative functors, which lead me to brain ache but enlightenment. So here’s my thoughts on how I got to understand these a little better.

All of the code is in Swift, so less clean than Haskell. I’m also only a about 6 weeks into Swift development so I probably haven’t got all of the idioms right. I’ve avoided the optional shorthand wherever possible here, preferring Optional<Type> over Type? because I believe the latter is hiding something that helps understand this code in the context of other generic classes.

It’s also long! I think it’s probably the longest blog post I’ve ever written but I found it interesting and useful, for myself, to write. If you’re one of those people who skip to the end of a book to find out whodunit then I’ve included

	man() {
	env \
	LESS_TERMCAP_mb=$(printf "\e[1;31m") \
	LESS_TERMCAP_md=$(printf "\e[1;31m") \
	LESS_TERMCAP_me=$(printf "\e[0m") \
	LESS_TERMCAP_se=$(printf "\e[0m") \
	LESS_TERMCAP_so=$(printf "\e[1;44;33m") \
	LESS_TERMCAP_ue=$(printf "\e[0m") \
	LESS_TERMCAP_us=$(printf "\e[1;32m") \
	man "$@"

	/****

	cxxforth: A Simple Forth Implementation in C++
	==============================================

	by Kristopher Johnson

	<https://github.com/kristopherjohnson/cxxforth>

	----

	COMPACT WIDTH (stacked view)

	- 320 x 568 pt
	-> iPhone 5, 5s
	- 320 x 768 pt
	-> iPad 9.7" Split Landscape 2/3 right
	- 320 x 834 pt
	-> iPad 10.5" Split Landscape 2/3 right
	- 320 x 1024 pt
	-> iPad 9.7" Split Portrait right

	#!/bin/sh

	# Current as working as of 2014/09/22
	# modified form of http://stackoverflow.com/questions/25056144/xcode-6-how-to-pick-signing-certificate-provisioning-profile-for-ad-hoc-distri
	# Xcode 6

	MYAPP="HELLOWORLD"
	OUTPUTDIR="$HOME/Desktop/$MYAPP"
	APPNAME="ANAppName"
	SCHEME="ASchemeName"

	struct User {
	let id: Int
	let name: String
	let email: String?
	}

	extension User: JSONDecodable {
	static func create(id: Int, name: String, email: String?) -> User {
	return User(id: id, name: name, email: email)
	}

	#pragma mark - WKUIDelegate

	- (void)webView:(WKWebView )webView runJavaScriptAlertPanelWithMessage:(NSString )message initiatedByFrame:(WKFrameInfo *)frame completionHandler:(void (^)())completionHandler
	{
	UIAlertController *alert = [UIAlertController alertControllerWithTitle:nil
	message:message
	preferredStyle:UIAlertControllerStyleAlert];

	[alert addAction:[UIAlertAction actionWithTitle:NSLocalizedString(@"OK", nil) style:UIAlertActionStyleCancel handler:^(UIAlertAction *action) {
	completionHandler();

	func unwrap<T1, T2>(optional1: T1?, optional2: T2?) -> (T1, T2)? {
	switch (optional1, optional2) {
	case let (.Some(value1), .Some(value2)):
	return (value1, value2)
	default:
	return nil
	}
	}

	func unwrap<T1, T2, T3>(optional1: T1?, optional2: T2?, optional3: T3?) -> (T1, T2, T3)? {